The present invention relates to a semiconductor light emitting device such as a semiconductor laser, a light emitting diode or the like and a fabrication method thereof, which uses a nitride-based compound semiconductor (a compound semiconductor of group III element(s) and nitrogen and the like), and is capable of emitting light in the blue color region required for an optical disk memory having a high memory density or improving delicacy of a laser beam printer. More particularly, the present invention relates to a semiconductor light emitting device such as a semiconductor laser and a fabrication method thereof capable of preventing warp in a wafer while suppressing the threading dislocation (defect) density of a nitride-based compound semiconductor layer as much as possible by employing epitaxial lateral overgrowth and of improving the electroluminescent properties.
A conventional light emitting diode (LED) or laser diode (LD) emitting light in a blue-emitting region has been fabricated by successively forming compound semiconductor of group III element nitrides on a sapphire substrate by Metal Organic Chemical Vapour Deposition (hereinafter referred to as MOCVD).
For example, a semiconductor laser capable of carrying out CW oscillation in a blue-emitting region is fabricated as shown in FIG. 10 by successively forming layers of group III element nitride type compound semiconductor on a sapphire substrate 21 by the MOCVD method; a GaN buffer layer 22, a contact layer 23 of an n-type GaN, an n-type clad layer 24 of Al0.12Ga0.88N, an n-type light guide layer 25 of GaN, an active layer 26 of an InGaN based (type) compound semiconductor with multiple quantum well structure, a p-type light guide layer 27 of a p-type GaN, a p-type clad layer 28 of a p-type Al0.12Ga0.88N, and a p-type contact layer 29 of a p-type GaN; etching some of the layered semiconductor layers as shown in FIG. 10 by, for example, dry etching to expose the n-type contact layer 23, and forming an n-side electrode 31 thereon and a p-side electrode 30 on the foregoing p-type contact layer 29, respectively. The portion of the p-side electrode 30 along the stripes is utilized as the light emitting part.
However, the sapphire substrate on which the nitride based compound layers are grown has considerably different lattice constant and thermal expansion coefficient from those of the nitride type compound semiconductor layers and the density of the threading 25 dislocation (TD) of the nitride based compound semiconductor layers grown thereon is as high as about 1xc3x97108cmxe2x88x922 to 1xc3x971010cmxe2x88x922 and the dislocation density is significantly high as compared with that, 1xc3x97102cmxe2x88x922, of compound semiconductor layers of the red-emitting type grown on GaAs substrate. In case of LEDs (light emitting diode), even if there occurs dislocation density about that level, a compound semiconductor is practically applicable, however in case of semiconductor lasers, if the dislocation density is especially high, the threshold current is increased, so that it is desired to lower the dislocation density at highest to about 1xc3x97107cmxe2x88x922 or lower in order to obtain a low threshold value and a long life. However, other than sapphire, any alternative substrate suitable for industrial use has not been found.
On the other hand, as a technique to lower the TD, crystal growth using ELO (epitaxial lateral overgrowth) has drawn attention as crystal growth methods, as disclosed in for example, xe2x80x9cThick GaN Epitaxial Growth with Low Dislocation Densityxe2x80x9d by Akira Usui et al. (Jpn. J. Apply. Phys., vol. 36, 1997, pp. 899-902) and xe2x80x9cELO growth of GaN by hydride VPE and MOVPExe2x80x9d by Sasaoka et al. (Jpn. J. Crystal Growth, vol. 25 no. 8, 1998, pp. 99-105).
These methods are methods including, for example, steps of putting a SiO2 mask 43 having opening parts 44 on a first GaN layer 42 on a sapphire substrate 41 and growing a second GaN layer 45 on the SiO2 mask 43 by selectively growing the layer in the lateral direction using the semiconductor layer exposed to the opening parts 44 as a seed as its sectional explanatory view is partially shown in FIG. 11 and prevent TD based on that a nitride type compound is easier to be grown in the lateral direction than in the vertical direction. The literature cited in the former journal discloses that a mirror face GaN layer free from cracks and having the dislocation density of lower than 6xc3x97107cmxe2x88x922 can be grown on a sapphire wafer with the diameter of 2 inch by the forgoing method.
However, in case of ELO growth, as shown in FIG. 11, although the second GaN layer 45 is so grown successively in the lateral direction from opening parts 44 in both sides formed at constant intervals in the mask layer 43 on the first GaN layer 42 as to meet in the center part of the mask layer 43, the second GaN layer 45 growing on the mask 43 tends to be lifted out of the mask 43 as it goes to the center part side and is grown while the crystallographic axis being curved to result in that the second GaN layer 45 cannot have a flat bottom face and surface. Therefore, as shown in FIG. 11, a void 46 is formed owing to the join of the second GaN layer 45 in the center part side of the mask while the second GaN layer 45 being lifted out and it is undesirable to fabricate a device using the resultant wafer. Such tendency further become significant if the mask width M becomes wide.
In order to avoid deterioration of the flatness, for example, as it can be understood from the case of the method cited in the literature of the above mentioned former journal where the mask width M of the SiO2 mask is 1 to 4 xcexcm and the cycle (M+W) is about 7 xcexcm, the mask width M is required to be narrow since the void 46 is easily formed if the mask width M is 3 xcexcm or wider. Moreover, as the width M becomes wider, the height and the size of the void become high and large and consequently,the flatness of the surface is deteriorated to result in inferiority of device properties. Further, even if the ultimate conditions in which no void 46 is formed and the flatness is barely kept, the dislocation density is increased in the join part in the center. Furthermore, the second GaN layer 45 growing in the opening parts 44 also has threading dislocation continuous as it is from the first GaN layer 42 in the vertical direction to become a region with a high dislocation density. Therefore, the continuous parts with a small dislocation density are only obtained in a half of the mask width and more precisely in portions excluding both end parts of the half and within about 1 xcexcm by width.
Nevertheless, in case of using the obtained wafer for a stripe type semiconductor laser, the light emitting part of the laser is only some region which is stripe-like and therefore it is supposed to be possible for the semiconductor laser to suppress the increase of threshold current value attributed to the crystal defects and the deterioration of electric operating properties of LD by lowering the dislocation density of the corresponding part of the semiconductor layer. In this case, as shown in FIG. 9, although it is effective to reciprocally form the opening parts 44 and mask layer 43 linearly in one direction, the dislocation density becomes high as described above in the both end parts and the center parts in the mask width and it is therefore preferable to use the portion excluding both end parts in a half of the mask width for a light emitting region with a stripe width of LD. Hence, supposing the stripe width of the LD to be 4 to 5 xcexcm, the mask width M is required to be at least 10 to 15 xcexcm and in this case, the GaN to be grown on the mask is required to be grown thicker than the mask width, that is 15 to 20 xcexcm.
As described before, in order to fabricate a semiconductor laser device with few crystal defects even only in the light emitting part of the device, it is required to form opening parts in the mask along the stripe direction in which the light emitting part of the device is formed and as shown in FIG. 9, the opening parts are formed in the mask only in one direction. On the other hand, there occurs no problem in case of a thin layered structure comprising a mask with a narrow width and the GaN layer with about 5 xcexcm thickness grown on the mask, however if the mask width is 10 xcexcm or wider and the thickness of a semiconductor layer to be layered thereon is 15 xcexcm or thicker, the warp of a wafer becomes significant attributed to the difference of the thermal expansion coefficient between the semiconductor layer and the substrate. In the case where the wafer is warped, uniform treatment in the wafer cannot be carried out during the wafer process, resulting in problems that the stripe width cannot be even, cracking easily takes place at the time of wrapping the wafer, and that properties are easy to be varied owing to the effect of the stress on the device by the warp.
On the other hand, if the thickness of the substrate is made as thick as about 700 xcexcm from the conventional thickness, about 330 xcexcm, such problems in the wafer process are eliminated, however there occurs another problem that the final polishing of the substrate before the substrate is made a chip requires a burdensome work and also if the substrate is polished the foregoing warping problem takes place. Further, Japanese Unexained Patent Publication No. Hei 11-186178 gazette discloses a method to prevent the wafer from warping owing to the difference of the thermal expansion coefficient from that of the substrate by employing the foregoing ELO growth and forming the grown regions like islands without carrying out epitaxial growth of a semiconductor layer on the entire surface of the substrate (no growth takes place by forming a large mask in a non-growth region). However, by this method, at least a half of the wafer is the non-growth region as examples show to leave a problem that the wafer is utilized considerably in vain.
The present invention has been developed in the above described situation and a first object of the present invention is to provide a nitride type compound semiconductor light emitting device with a high light emitting efficiency by selectively forming a nitride type compound semiconductor layer with flatness in a wide range on a mask of SiO2 or the like while suppressing the dislocation density.
Another object of the present invention is to provide a semiconductor laser capable of providing high outputs by suppressing the dislocation density of the active layer in at least stripe-like light emitting region and lowering the threshold current value in the case where the light emitting region can be restricted within the stripe-like part just as the case of a semiconductor laser.
Further another object of the present invention is to provide a semiconductor laser with a structure possible to prevent warp of a wafer while suppressing the dislocation density of the active layer in such stripe-like light emitting region.
Further another object of the present invention is to provide a method for fabricating a semiconductor light emitting device by which the warp of a wafer is suppressed to the extent that there occurs no problem in the device properties and the handling of the wafer while the threading dislocation density of the entire wafer being suppressed by widening the mask width in the case where a semiconductor layer is selectively grown on the mask by ELO growth.
Inventors of the present invention have enthusiastically investigated in order to solve a problem that a growing nitride type compound semiconductor layer is grown while the crystallographic axis being warped more upward as it goes closer to the center part of a mask to leave a void in the periphery of the center part in case of selective growth of the semiconductor layer on the mask in the lateral direction and that the void becomes bigger as the mask width is wider to inhibit growth of the flat semiconductor layer, and inventors have found the reason why the upward warping of the crystallographic axis of the growing semiconductor layer is getting more significant as the semiconductor layer is grown closer to the center part side of the mask is attributed to the contact stress affecting the contact parts of the semiconductor layer and the mask layer. Inventors have also found that a flat semiconductor layer with a small dislocation density can be grown without being accompanied with the warp of the crystallographic axis by parting the contact parts and contact stress is prevented from affecting.
A semiconductor light emitting device according to the present invention comprises a substrate, a mask layer having opening parts and formed directly on the substrate or on a layer formed on the substrate, a nitride type compound semiconductor layer selectively grown in the lateral direction on the mask from the opening parts, and a semiconductor layered part comprising nitride type compound semiconductor layers so formed on the nitride type compound semiconductor layer as to form light emitting layer, wherein the mask layer is provided with at least one recessed part on the upper face side, or the foregoing second nitride type compound semiconductor layer has a flat face in the bottom face side and is so grown as form an approximately parallel gap between to the bottom face of the second nitride type compound semiconductor layer and the mask layer.
In this case, the nitride type compound semiconductor denotes a semiconductor of a compound of group III elements such as Ga, Al, In and the like, and N and other elements of group V elements. Consequently, the compound semiconductor means N-containing compound semiconductor with properly changed mixed crystal ratio of the group III elements and of the group V elements, such as AlGaN type compound in which the composition ratios of Al and Ga can be changed, InGaN type compound in which the composition ratio of In and Ga can be changed, other than GaN. Further, the mask layer means a layer made of a material, for example, SiO2, on whose surface an epitaxial growth of a nitride type compound semiconductor layer cannot directly be carried out in case of trying the epitaxial growth.
According to such a structure, since recessed parts are formed in the mask layer to be an underlayer of the second nitride type compound semiconductor layer selectively grown in the lateral direction, or the second nitride type compound semiconductor layer is formed as to keep a gap to the mask layer, a second gallium nitride type compound semiconductor layer to be grown is not affected with the stress from the mask layer. As a result, the crystallographic axis of the second gallium nitride type compound semiconductor layer is prevented from upward warping as the compound semiconductor layer is growing in the lateral direction and the semiconductor layer is straightly grown in the lateral direction in a wide width to obtain the second nitride type compound semiconductor layer excellent in flatness and having an extremely low dislocation density. Further, since the semiconductor layered part of the nitride type compound layered thereon is grown on the semiconductor layer with a low dislocation density, it is made possible to form the semiconductor layered part with excellent flatness and a low dislocation density.
The layer to be formed on the foregoing substrate may be a first nitride type compound semiconductor layer and the foregoing nitride type compound semiconductor layer to be formed on the foregoing mask layer may be a second nitride type compound semiconductor layer.
A semiconductor laser of the present invention is the foregoing nitride type compound semiconductor light emitting device in the above described in claim 1 or 2, wherein the semiconductor layered part is laminated so as to constitute a semiconductor laser structure, the mask layer has a part extended like stripe by being sandwiched between neighboring opening parts, the recessed part or the gap is formed in a prescribed width along the stripe in the part extended like stripe, and the semiconductor layered part is so formed that a current injection region in stripe is formed in a constant width within a half of the prescribed width. By constituting such a structure, without requiring semiconductor layers with a low dislocation density to be formed in a wide range, the semiconductor layered part necessary in light emitting region in stripe comprises only layers with a low dislocation density and can be formed as to have excellent flatness and it is thus made possible to obtain a semiconductor laser with a small threshold current value, a high output and excellent reliability.
Further, inventors of the present invention have made investigation in order to eliminate the effect of the wafer"" warp attributed to the wide width of the mask and the thick thickness of the nitride type compound semiconductor layer grown thereon in case of selective growth (ELO growth) of the nitride type compound semiconductor layer on the mask in the lateral direction and consequently found that if opening parts of the mask layer are formed continuously only in one direction, the warp takes place in a constant direction in relation to the opening parts and that, on the contrary, if most parts of the opening parts are dispersed or the opening parts are not formed to be continuous only in one direction, the warp can significantly be suppressed even if the thickness of the growing layer is made thick by widening the mask width or even if long and straightly linear opening parts are formed.
A method for manufacturing a semiconductor light emitting device according to the present invention comprises the steps of; forming a mask layer, on which a nitride type compound semiconductor layer cannot be formed directly, either directly on the surface of a wafer-type substrate or on a layer formed on the substrate; forming opening parts for exposing seeds to grow nitride type compound semiconductor layer layers in the mask in a manner that the opening parts are not arranged only continuous in a single direction in the entire surface of the foregoing wafer-type substrate; forming a nitride type compound semiconductor layer on the entire surface of the foregoing wafer-type substrate by selectively growing the layer on the foregoing mask layer from the opening parts in the lateral direction; forming a semiconductor layered part comprising nitride type compound semiconductor layers as to form light emitting layer on the foregoing nitride type compound semiconductor layer; and producing chips from the resultant wafer type substrate.
According to the present method, since the opening parts of the mask layer are not formed continuously only in one direction, the stress owing to the difference of the thermal expansion coefficient between the substrate and the semiconductor layers grown thereon does not affect only in one direction but evenly applied in every direction to result in prevention of significant warp in only one direction. Consequently, treatment unevenness in wafer processing process and cracking of the wafer can be avoided to considerably improve the quality of the resultant product.
Most of the foregoing opening parts are formed to be a rectangular or hexagonal shape in a plan view, so that the length in the growth direction can be easy to be adjusted even if the speed of the crystal growth in the lateral direction is different between the direction perpendicular to the A face and the direction perpendicular to the M face in case of the rectangular shape and the growth speeds in the respective directions can be made equal one another due to that a GaN type compound is of a hexagonal system in case of the hexagonal shape, so that the growth in the lateral direction can be made even. That most of the opening parts are formed to be a rectangular or hexagonal shape in this case includes that straightly linear opening parts formed partially, for example, in the peripheries of the stripe-like light emitting parts of an LD.
Another embodiment of the semiconductor laser according to the present invention is a semiconductor laser comprising; a substrate, a mask layer having opening parts and formed directly on the substrate or on a layer formed on the substrate, a nitride type compound semiconductor layer selectively grown in the lateral direction on the mask layer from the opening parts, and a semiconductor layered part comprising nitride type compound semiconductor layers so formed on the nitride type compound semiconductor layer as to form a light emitting layer having a stripe type light emitting part, wherein the mask layer has a part extended in the entire chip length with no opening part transversely crossing the lower side of the stripe type light emitting part and has the opening parts dispersedly in the parts other than the lower side of the stripe type light emitting part.
Owing to such a structure, the stripe type light emitting part of the semiconductor laser are composed of only parts with extremely few crystal defects and since the opening parts of the mask layer are not formed only in one direction, the warp of the wafer can be suppressed to a significantly low level and the production yield is improved and at the same time the obtained semiconductor laser is provided with excellent properties and a low threshold current.
More practically, the opening parts of the mask layer are formed linearly along the stripe type light emitting part in the portions adjacent to the stripe type light emitting part and formed in a matrix-like form or randomly in the portions other than the portions adjacent to the stripe type light emitting part or the mask layer is formed into a shape symmetric three times by layering a first pattern having linear opening parts, a second pattern obtained by rotating the first pattern at 60xc2x0, and a third pattern obtained by rotating the first pattern at 120xc2x0, so that the opening parts of the mask layer are not restricted only to those continuous only in one direction and the semiconductor laser having the stripe type light emitting part can be formed by semiconductor layers with a low dislocation density entirely in the stripe type light emitting part.